As shown in FIG. 2, a prior art holder for an end mill, a drill and other cutting tools for drilling and boring comprises a holder body 1 which can be inserted in a spindle of a machine tool and a collet 2 inserted in the body 1 so as to be aligned with its axis. After inserting a cutting tool 3 into the collet 2, the collet is pulled backwards along a tapered portion 4, so that its diameter will be reduced so as to chuck the cutting tool 3.
As means for moving the collet 2 back and forth, it is known (a) to draw a drawing screw 5 in threaded engagement with the collet 2 by means of a rod from the side of the machine tool (direct drawing method), (b) to screw a locknut 6 on the holder body 1 as shown by chain lines and tighten it and (c) to move a pin 7 extending transversely through the drawing screw 5 in an axial direction by turning a clamping collar 8, as shown by solid lines in FIG. 2.
In the drawing, numeral 9 designates through holes in the holder body 1 through which the pin 7 extends. They have a width (in the circumferential direction of the body 1) which is equal to the diameter of the pin 7 and have a predetermined length (in the axial direction of the body 1). The pin 7 is received in the through holes 9 so as to be movable axially but not circumferentially with respect to the holder. Numeral 10 designates a screw ring in which the pin 7 is fitted. The screw ring 10 is in threaded engagement with female threads 8a on the clamping collar 8. As the clamping collar 8 is turned, the pin 7 is moved right and left by the screw ring 10. Numeral 11 designates a bearing.
A spring collet 2a as shown in FIGS. 7a and 7b is chiefly used with the above described collet for moving method (c). It is formed with slits 20 extending from both end faces thereof. It can hold a cutting tool with sufficient force and accuracy (degree of runout of the cutting tool being chucked).
But when this collet 2a is pulled to reduce its diameter, its segments 2a.sub.1 and 2a.sub.2 divided by respective slits 20 so as to be connected with each other at their front ends will undergo at their front ends a shrinking force F.sub.1 in the direction of the arrows in FIG. 8a. On the other hand, the segments 2a.sub.1 and 2a.sub.2, which are connected with each other at their rear ends, will undergo at their rear ends a shrinking force F.sub.2 in the direction of the arrows in FIG. 8b. In other words, the clamping forces F.sub.1 and F.sub.2, which differ in direction from each other, act on the segment 2a.sub.1 at its front and rear ends, respectively. This will cause the segment 2a.sub.1 to be twisted in the direction of its length. All the segments 2a.sub.1 to 2a.sub.8 will be subjected to the same kind of twist.
The rate of twist increases with an increase in the shrinkage of diameter. The larger the rate of twist, the more the accuracy will decrease. Thus with this type of collet 2a, it is necessary to set the shrinkage rate (rate of reduction in diameter) at about 0.2 mm in order to give a satisfactorily accurate finish in end milling by use of the cutting tool 3.
The abovementioned problem could be solved by increasing the number of slits. But the structure is limited to 16 slits (8 slits extending from each end) in view of the strength of the entire collet.
FIG. 9 shows a collet 2b which is mainly employed with the moving methods (a) and (b). It comprises a trucated conical portion 21 and a cylindrical portion 22 integral with each other at the small-diameter end of the portion 21. The collet 2b is formed with a plurality of slits 20 arranged at equal angular intervals from one another and extending axially and inwardly from the tool receiving opening a for inserting a cutting tool 3. The number of slits 20 is usually 3, 4 or 6.
But if this collet 2b is used to chuck a cutting tool 3 having a diameter considerably smaller than the inner diameter of the collet 2b as shown in FIG. 10, there will be a gap between the inner surface of the truncated conical portion 21 and the outer surface of the cutting tool 3 as shown in FIG. 10b. This makes it difficult to grip the cutting tool with sufficient force and accuracy. Thus if the collet 2b is used for end milling, the shrinkage rate has to be limited to about 0.2 mm in order to give a satisfactory finish.
FIG. 11 shows another known collet 2c having a plurality of segments S which are not connected with the adjacent ones as with the collets 2a and 2b. Instead, they are bound together by means of synthetic resin c. But the use of synthetic resin c will lower the accuracy of chucking.
It is an object of the present invention to provde a method of determining a minimum mumber of segments for a collet which in view of the abovesaid shortcomings, can grip a cutting tool with sufficient force and accuracy and over a large range of tool diameters.